1. Introduction
As a fundamental research topic, Super-Resolution (SR) attracts long-standing substantial interest, which targets high-resolution (HR) image reconstruction from a single or a sequence of low-resolution (LR) observations. In recent years, we have witnessed the prosperity of Single Image Super-Resolution (SISR), e.g., SRCNN [7], EDSR [19], SRGAN [16], RDN [33] and ESRGAN [28]. Nevertheless, SISR intrinsically suffers from a limited capacity of restoring details from only one LR image, typically yielding oversmooth LR predictions, especially for large-scale factors. With real detailed sub-pixel displacement information, Multi-Frame Super-Resolution (MFSR) [31], [1], [2], [21], [20] provides a promising potential to reconstruct the high-quality image from multiple LR counterparts, which is valuable for many sensitive realistic applications, e.g., medical imaging, and remote satellite sensing.